I tried to build my girl up a nice and light little single speed bike that'd be easy to carry up the 4 floors to her apartment.. but nooooo she likes her 50lb schwinn women's cruiser that I need to carry up the stairs for her. life just isn't fair. But yea 5'4" you've got your options pretty open.. Not until you get down to 5'1" and below is it a major pain in the ass. I'd think the heidi would be too small but the standard IRO mark V comes small enough as someone said and you can customize it which is good for making it more "girly"

But I'd definitely recommend testing the waters.. having her borrow a friends bike and going on a ride or two with her see how she likes it.. Hey, even see how YOU like riding with her.. Because that'll be sure to be a messy situation if you drop a few hundred on a bike for her and she rides it once.